Embodiments of the present invention relate to writing a circuit design pattern on a substrate with a shaped electron beam.
Pattern generators are used to write patterns on substrates, such as masks for semiconductor fabrication, semiconductor wafers, displays and circuit boards, magnetic bubble fabrication, and optical data storage media fabrication. Patterned lithographic masks are used to transfer a circuit design pattern into a substrate, such as a semiconductor wafer or dielectric to fabricate integrated circuits, printed circuits (PCB), displays, and other patterned boards. A typical process for fabricating a lithographic mask includes, for example, (i) forming a layer of a metal-containing material on a radiation permeable plate, (ii) forming a resist layer on the metal-containing layer to create a blank lithographic mask, the resist layer being sensitive to a charged electron beam such as an electron or ion beam, (iii) writing a pattern on the mask by selectively exposing the blank lithographic mask to the modulated electron beam, (iv) developing the exposed material to reveal a pattern of features, (v) etching the revealed portions of the metal-containing material between the resist features to transfer the pattern captured in the resist features into the metal-containing material, and (vi) stripping residual resist from the lithographic mask.
In electron beam pattern generation, the circuit design pattern is written by selectively exposing an electron sensitive resist on the substrate to a modulated electron beam. The electron beam is formed in a beam column having discrete components that generate, focus, blank and deflect the electrons to write a pattern on a substrate. Conventional electron beam columns are for example described in U.S. Pat. No. 6,262,429 to Rishton et al.; U.S. Pat. No. 5,876,902 to Veneklasen et al.; U.S. Pat. No. 3,900,737 to Collier et al.; and U.S. Pat. No. 4,243,866 to Pfeiffer et al.; all of which are incorporated herein by reference in their entireties. The modulated electron beam is moved and flashed across the substrate using a scanning system, such as for example, raster, vector, or hybrid raster-vector scanning. The beam and substrate are moved relative to one another so that the beam traverses across the substrate in linear strip-wise motion (e.g. raster beam scanning), in vector-based steps (vector scanning), or in a combination of vector and raster scanning. For example, in one raster scan method, called raster Gaussian beam scanning (RGB),
A preferred writing method, commonly known as shaped beam writing, a variable shaped beam is moved directly to locations above a substrate specified by vector coordinates that can be independently derived or localized along a raster scan, and flashed once over those locations. In this technique, the circuit design pattern is divided into a series of geometric primitives which are then refined to rectangles, parallelogram, and triangle flash shapes associated with location coordinates. The electron beam is then moved directly to the location coordinates that fall within a raster scan and flashed to expose the location site with the desired shaped electron beam flash. The beam is shaped for each flash using beam shaping beam shapers, which have apertures with dimensions of typically larger than 1 μm. The shaped beam provides flash profiles having sharper edges than the Gaussian curve edges of Gaussian beams, providing higher resolution and better critical dimension uniformity. Also, the beam traverses directly to the area to be written skipping unexposed areas to reduce total exposure time by a percentage corresponding to that of unwritten/written area. It is desirable to have higher resolution coupled with faster writing speeds to write circuit design patterns having features with increasingly small dimensions.
In the shaped electron beam column, the electron beam is generated from an electron source, which provides a stream of electrons. Conventional shaped beam columns often use thermionic electron sources typically comprising lanthanum hexaboride (LaB6), an exemplary version being shown in FIG. 1A. The thermionic source 10 comprises a rounded tip 12, which is heated to a temperature at which the electrons have sufficient energy to overcome the work function barrier of the LaB6 conductor to escape from the tip. The emitted electrons are accelerated by a Wehnelt anode 11, which is typically, and distance of several millimeters. The rounded tip 12 has, for example, a relatively large radius of approximately 5 μm, and produces a large and uniform electron distribution 14 with adequate brightness to illuminate the apertures used to shape the beam. The large illumination area also allows the apertures to selectively pass through only the central region of the beam to provide a more uniform electron density and exclude Gaussian curved edges. However, while the thermionic source 10 works well for conventional shaped electron beam writing, they are optimal for writing ever smaller features which now are smaller than 90 nm in dimensions.
Thermal field emission (TFE) electron sources 15 are typically not used for shaped beam electron columns. An exemplary version of a TFE electron source 15, as shown in FIG. 1B, comprises a narrow needle tip 16 of tungsten, which typically has a radius of approximately 0.3 to 1 μm (about 1/10 the size of the radius of the thermionic source 10). The needle tip 16 is heated to temperatures of about 1800 K, while an electric field is applied using a suppressor 20 and an extractor 22 spaced apart distance of several hundred microns (as opposed to millimeters with the thermionic source) that is sufficiently strong to cause electrons to tunnel through the barrier and be emitted as a narrow beam 18. The tungsten tip 16 is usually coated with a layer of zirconium oxide to reduce the work function barrier, and a heated reservoir of zirconium oxide (not shown) continuously replenishes material evaporated from the tip 16. The TFE source 16 features a higher brightness and associated depth of field than the thermionic sources, a small virtual source size, and a moderate energy spread, making is more desirable to achieve the higher beam currents which are needed for efficiently writing the ever smaller features. The small angular intensity of the TFE source 16 means that a relatively large angle of emission must be accepted from the gun to capture a certain total current. Unless large (>100 μm) shape apertures 126 are used, this large angle must be reduced by a large magnification M in the upper column. The combination of the large gun angle and large M increase the effect of spherical aberration on illumination uniformity at the upper shape.
Thermionic sources 10 have been used to generate shaped electron beams using Köhler illumination mode optics, as for example disclosed in Komagata et al., SPIE 2096 (1997) pp 125-136, which is incorporated herein by reference in its entirety. It is desirable to be able to use TFE sources 16 with Köhler illumination because the TFE sources 16 provide higher illumination brightness allowing for the possibility of higher beam currents in small shaped beams. However, the narrow electron beam 18 from the TFE source 16 has to be magnified to be significantly larger than the beam shaping apertures to be used with Köhler illumination mode optics. Conventional electrostatically focused particle gun which use TFE sources 16 have a large spherical aberration coefficient, and upon magnification, the spherical aberration would grow as the product of the coefficient times the third power of the illumination angle times the magnification. Thus, the use of TFE sources 16 with critical or Köhler illumination of a beam shaping aperture in a conventional optics would lead to excessively large spherical aberration due to the magnification required. Aforementioned U.S. Pat. No. 6,262,429 to Rishton describes shadow projection shaped-beam optics in order to avoid magnifying a small TFE source; however, shadow projection optics have undesirable properties including lack of focus of the beam spot onto the image plane. Thus, it is desirable to have a shaped electron beam column capable of using a TFE electron source 16 in combination with Köhler illumination optics.
In shaped electron beam columns, the electron beam image to be projected onto the substrate is created by deflecting the image of an upper aperture onto lower apertures having different shapes to create a composite image having the desired beam shape. The beam shaping module performs the functions of illuminating the apertures, selecting a beam current, imaging the upper aperture on the lower aperture, projecting a demagnified image of the apertures into the substrate plane, and selecting a suitable beam aperture in the substrate plane. In conventional electron beam systems, these parameters are coupled. For example, the condition of imaging the first aperture upon the second aperture, and having a fixed location of the crossover between both apertures (which defines the illumination angle of the apertures) fixes beam current and final aperture angle. However, it is often desirable to be able to select different electron beam paths to adapt to different illumination requirements in the substrate plane to compensate for changes in beam illumination characteristics and associated changes in beam crossover points.
Another problem with conventional shaped beam columns arises because the excitation of the focus lens in the column has to be changed dynamically while a focused charged electron beam is deflected over large angles across the substrate because the distance from the deflection pivot point to the flat object changes with the angle. For raster or vector scanning systems, the beam deflection speed usually is so high that the main objective lens, which is generally magnetic, cannot be adapted fast enough due to its high inductivity and the creation of eddy currents. Thus, additional small magnetic coils are inserted into the objective lens for better dynamic focus control. However, the upper frequency of these small magnetic coils is also limited by eddy currents and inductivity, unless they are isolated magnetically and electrically from their surroundings, for example by ferrite shields.
Another dynamic focusing solution involves use of an electrostatic focusing lens, as for example described in T. Hosokawa et al, JVST B1(4), 1983, P1293ff, which is also incorporated herein by reference in its entirety. Electrostatic lenses generally have even better high frequency behavior than small magnetic coils. However, electrostatic focusing lenses general consist of three round concentric electrodes, which add space requirements and thereby increase total column length. As the column length increases, the electron beam is subject to increased beam broadening effects due to electron-electron interactions and sensitivity to noise, which in turn reduces pattern-writing resolution. Thus, it is desirable to have a more compact electrostatic focusing lens system, which can rapidly deflect the electron beam while reducing electron beam column length.
Thus it is desirable to have a pattern generation system capable of using a field emission source to write patterns with a shaped beam writing method. It is further desirable to be able to use field emission sources to produce a large wide area beam for illuminating apertures without excessive spherical aberration or magnification problems. It is also desirable to use the field emission source with Köhler illumination of a beam-shaping aperture. It is also desirable to have a more compact electrostatic focusing lens system, which can rapidly deflect the electron beam while reducing electron beam column length to maintain beam spot resolution.